This invention relates to a method and apparatus for controlling an extrusion process such that the weight/meter of extrudate can be accurately controlled.
The invention is especially concerned with the extrusion of a tubular covering of rubber or thermoplastics material on to a core, in which the linear speed of the extrudate as finally formed is equal to the speed at which the core passes through the machine. When extruding sections not formed as tubular coverings on a core, the linear speed of travel of the extrudate as finally formed is considered to be the speed at which it is taken up, e.g. by a capstan, an endless belt haul-off, or a take-up drum, from the extrusion machine after it has been cooled. This speed and the core speed when tubular coverings are being formed on a core will herein each be referred to generically as the "line speed".
It is known to attempt to control the weight/meter of extrudate on a cable by measuring the diameter of a cable and varying the extrusion process to attempt to maintain the diameter constant. This suffers from the disadvantage that the diameter of a cable is not easy to accurately determine until the extrudate has been cooled. Furthermore where a core of non-circular cross-section is employed the diameter of the cable produced may not be an accurate reflection of the weight/meter of extrudate applied thereto.
It is therefore proposed to provide an extrusion process in which the weight/meter of extrudate can be more accurately controlled.
Accordingly a method of controlling an extrusion process comprises the steps of feeding the material to be extruded from a supply station to an extruder;
repeatedly weighing the supply station to calculate the through-put of the extruder;
calculating the initial line speed necessary to produce a predetermined weight/meter of extrudate;
adjusting the line speed directly to said initial line speed;
subsequently increasing the throughput of the extruder and the line speed simultaneously, such that the weight/meter of the extrudate is maintained substantially constant, until any one of a plurality of measured parameters reaches a predetermined maximum value; and
thereafter adjusting the line speed or extruder throughput in response to the calculated throughput of the extruder such as to maintain the weight/meter of the extrudate substantially constant at said predetermined weight/meter.
Conveniently the supply station is weighed by a load cell, typically by being freely suspended therefrom. Repeated measurement of the combined weight of the supply station and material therein will allow calculation of the weight of material leaving the supply station and hence the throughput of the extruder. Measurement of the weight of material entering the extruder is a direct indication of the performance of the extruder. Alternative systems which monitor the volume of material entering the extruder are inherently less accurate due to the property that the flow of material from the supply station is not regular.
The supply station is preferably a container in which the material to be extruded is held. Alternatively, where the material to be extruded is in strip form rather than in the form of granules, powder etc, the supply station is conveniently a platform such as a pallet or the like.
It is envisaged that the feed to the extruder will be such that it is choke fed and the throughput of the extruder varied by varying the screw speed thereof. However it is equally conceivable that the extruder may be starve fed and its throughput controlled, additionally or alternatively to the screw speed, by varying the amount of material fed to the extruder. It will be appreciated that whether the extruder is choke fed and its throughput is monitored, or whether the extruder is starve fed and its throughput is metered, repeated weighing of the supply station will allow the throughput to be calculated.
Conveniently the throughput of the extruder and the line speed are simultaneously increased in an incremental manner, separated by periods in which only one of the throughput or line speed is varied such as to maintain the weight/meter of the extrudate substantially at the predetermined weight/meter. This incremental increase allows any discrepancy which may be introduced into the weight/meter of the extrudate by the simultaneous increase of the throughput and line speed to be corrected. Each increment is preferably preset to subsist for a predetermined time period. Additionally the time periods of respective increments conveniently vary according to a preset progression. For example a decreasing incremental length is envisaged such that more frequent corrective action is taken at relatively higher line speeds.
Conveniently the method includes the additional step of reducing the extruder throughput and the line speed, once at least one of said measured parameters has reached its predetermined maximum value, until said at least one parameter again falls below the predetermined maximum value. This "backing-off" of the production rate is to avoid a situation in which the production rate is maintained whilst said parameter continues to rise above its predetermined maximum value. By reducing the throughput and line speed until the parameter once again falls below maximum, the possibility of the parameter considerably overshooting its maximum is removed.
The measured parameters conceivably include the line speed, the screw speed of the extruder, the load current of the motor driving the extruder screw, the melt pressure at the extruder head and the temperature of the extrudate emerging from the extruder. Other parameters may be measured and monitored where appropriate to the manufacturing process.
The method of the present invention is particularly suited to the production of variable density extrudate. By the term variable density extrudate is herein meant material which is expanded by a gas, normally nitrogen, to form an extrudate which is foamed, or is solid material which has cavities or segments therein. The gas is either injected into the extrudate or is produced chemically by the addition of a gas producing compound to the material to be extruded. The expansion continues until the extrudate is stabilized by cooling at which point the gas produced cavities are "frozen" into the extrudate.
Accordingly the method conveniently includes the steps of measuring at least one additional parameter which is either a suitable dimension or the capacitance of the stabilized extrudate, and adjusting the temperature profile of the extrudate in response to said measured additional parameter in order to maintain said additional parameter substantially constant. The suitable dimension is typically the diameter of the extrudate. The temperature profile of the extrudate is conveniently adjusted by changing the longitudinal position of a cooling means, adapted to stabilize the extrudate. The cooling means is conceivably a quenching bath containing coolant such as water. Alternatively cooling is achieved by means of one or more jets adapted to spray air, water or other coolant on to the extrudate.
The invention further resides in an extrusion apparatus controlled by the method described herein. In particular the extrusion apparatus comprises an extruder; a supply station adapted to support material to be extruded; supply means adapted to feed the material to the extruder; weighing means for repeated weighing of the supply station; drive means for controlling the line speed; means for measuring the line speed; sensors for sensing a plurality of parameters; and electronic processing means adapted to:
I. receive repeated signals from the weighing means and calculate the throughput of the extruder,
II. calculate an initial line speed necessary to produce a predetermined weight/meter of extrudate,
III. supply signals to the drive means to adjust the line speed to said initial line speed,
IV. compare the plurality of sensed parameters against predetermined maximum values for these parameters, and if all are below said maximum values supply signals to the drive means and to the extruder to increase the line speed and the throughput of the extruder simultaneously, such that the weight/meter of the extrudate is maintained substantially constant, and
V. supply signals to the drive means, once at least one of said sensed parameters is at its maximum value, to adjust the line speed or extruder throughput in response to the calculated throughput of the extruder such as to maintain the weight/meter of the extrudate substantially constant at said predetermined weight/meter.
Conveniently the electronic processing means is a microprocessor or microcomputer.